This invention is directed toward a method and apparatus for determining the state-of-charge and capacity of an electrochemical battery system. More precisely, a method and apparatus particularly suitable for use in systems characterized by widely varying discharge currents and discharge rates, widely varying operating temperatures, deep discharges, and frequent recharging.
In application to electric vehicles, it is particularly desirable to indicate to a vehicle operator the useful remaining energy of the motive battery system and/or range available therefrom in a fashion much analogous to a fuel gage and/or range gage in a conventional internal combustion powered vehicle. This information will of course inform the operator when the capacity of the motive battery system is nearing depletion thereby necessitating a recharge in order to avoid operator inconvenience and/or irreversible battery damage. A further need exists for such information at the system level so that efficient recharging can be performed based upon the motive battery system present state-of-charge and capacity.
Prior art ampere-hour integration techniques have been used to indicate battery state-of-charge but fall short of accurately predicting state-of-charge in vehicle specific applications which are characterized by dynamic battery capacities related to varying discharge current magnitude and rates as well as variable temperature operating conditions. Prior art battery terminal voltage techniques have also been utilized to indicate battery state-of-charge but are expensive due to the precision voltage measurements required and suffer from implementation problems into a dynamic system since they typically require open circuit, near zero current, voltage measurements thereby requiring operative interruption. Additionally, after termination or interruption of current flow for terminal voltage measurements, the polarization voltage decay time constant would be on a much greater order of magnitude than any practical period through which a current interruption could be tolerated during any dynamic electrical vehicle operation or expedient recharging cycle, thereby resulting in inaccuracies attributable thereto.